The present invention relates to a blank and a method of manufacturing a rack shaft using the same.
Conventionally, an apparatus for manufacturing a rack shaft has a plurality of rack teeth molded into a round rod-shaped blank by forging. The plurality of rack teeth is molded to be aligned in an axial direction of the blank. In recent years, manufacturing apparatuses for performing near net shape forging in which a shape after forging the blank becomes a shape close to a final product have been suggested.
A manufacturing apparatus described in Japanese National Phase Laid-Open Patent Publication No. 2013-501621 includes a first gripping portion, a second gripping portion, and a tooth forging portion. The first gripping portion has a first cylindrical inner circumferential surface coming into contact with a first shaft portion of the blank. The second gripping portion has a second cylindrical inner circumferential surface coming into contact with a second shaft portion of the blank. The teeth forging portion is formed to define a space having substantially the same shape as a final shape of the teeth portion of the rack shaft formed with the rack teeth. In this manufacturing apparatus, the first shaft portion is gripped by the first gripping portion, and the second shaft portion is gripped by the second gripping portion. In this state, a machining target portion of the blank provided between the first shaft portion and the second shaft portion is located in a space of the teeth forging portion. Moreover, the blank is plastically deformed by the teeth forging portion, and thus, the rack teeth are molded.
When the blank is plastically deformed by forging as described above, the material of the blank flows. Specifically, as illustrated in FIG. 9, first, teeth 92 of the teeth forging portion are pressed against a machining target portion 91 of the blank. Then, in the machining target portion 91, a site coming into contact with each of machining teeth 93 is depressed. Thus, as illustrated by two-dot chain line in FIG. 9, the material of the machining target portion 91 swells to enter a space S between the adjacent machining teeth 93. In this way, the shape of the teeth 92 is transferred into the machining target portion 91 to mold the rack teeth. A thick line in FIG. 9 schematically illustrates the flow of the material.
Thus, at the first end portion of the machining target portion 91 close to the first shaft portion, when pressed by the machining teeth 93, the material that does not enter the space S may flow into the first shaft portion. Similarly, also at the second end portion of the machining target portion 91 close to the second shaft portion, when pressed by the machining teeth 93, the material that does not enter the space S may flow into the second shaft portion. For that reason, at both end portions of the machining target portion 91 close to the first and second shaft portions, the material easily decreases as compared to an intermediate portion in the axial direction of the machining target portion 91. That is, when forging the machining target portion 91, at both end portions of the machining target portion 91 close to the first and second shaft portions, a filling rate of the material within the space of the teeth forging portion easily decreases. As a result, at both end portions of the machining target portion 91 close to the first and second shaft portions, the material may not reach the bottom surface of the teeth 92. For that reason, there is a possibility that the rack shaft may be molded in a state in which a part of the rack teeth are lacked.